1. Technical Field
The invention is a snap locking angle adjustable device, particularly applicable to adjustable carpenter's squares—squares in which the angle between the blade (or tongue) and the handle (or stock) is adjustable to a number of predetermined positions. An equivalent name is adjustable builder's square. The field of use is in particular construction work such as building houses. The invention may also be categorized as a foldable square: a square having a blade that may be folded into the handle, for example to facilitate putting the square into a pocket. In a wider sense, the invention is a snap locking angle adjustable device, which may be: a component of or a complete carpenter's square, or: a component in a mitre saw or other tool that, from time to time, is being adjusted to predetermined angles.
2. Technical Problem Solved by the Invention
The invention solves problem with critical manufacturing steps for angle adjustable carpenter's squares. This specifically applies to the product “Quattro” from Nobex AB and the square documented in SE-451,443. As part of the manufacturing, four steel balls are pressed into four holes of an aluminum profile that is to become the handle of the square. Another process step is to punch 8 holes into a steel sheet that is to become the blade of the square. The holes' diameters and positions and the steel balls' settling positions, are critical parameters that determine the angle precision of the square. In other words, the angle precision depends both on processing (hole punching) the handle and the blade and on mounting several parts (steel balls) into precise positions. That is one aspect of the problem; the following is another aspect. In the case of the blade having eight holes, the blade locks into position at angles spaced with 45° interval. In order to obtain a 22.5° spacing, 16 holes would be needed. However, compared to holes, this would yield much thinner metal bridges between neighbouring holes—much thinner, because the holes are positioned in a row along a circle with a radius that can not be much larger than 15 mm, and because the holes have a certain feasible minimum diameter—thus causing difficulties to fabricate the square with maintained angle precision.
3. Description of Related Art
The product “Quattro” from Nobex AB and the square documented in SE-451,443 are examples of adjustable carpenter's squares. Snap locking is achieved by the following means. The square's blade has a number of holes concentrically placed around the (blade's and the handle's common) pivot axis. Similarly, the square's handle has a number of steel balls concentrically placed around the pivot axis; the steel balls protrude in axial direction above a plane normal to the pivot axis. A strong metal spring presses the blade against the handle in the axial direction. At certain angles between the handle and the blade, the steel balls align with the holes, causing the blade and the handle to snap lock.
A measuring instrument is described in U.S. Pat. No. 5,020,233. The member arms (corresponding to the blade and the handle of a carpenter's square) are formed in plastics or other stiff but elastically distortable material. The member arms are attached to each other such that a tension is created in the elastic material, thus giving rise to a spring force—analogous to the force created by the metal spring of the carpenter's squares above—that presses the member arms axially against each other. Detents and protrusions in the material, concentrically around the pivot axis, of each member arm—analogous to the steel balls and holes—are responsible for a snap locking function.
There are also adjustable squares having other locking mechanisms. First, there are a number of squares characterized by radial alignment structures (not axial, as the steel balls and the other alignment structures above). The following patents describe such squares:                FR-761,096        U.S. Pat. No. 5,839,201        U.S. Pat. No. 6,141,882        U.S. Pat. No. 304,196        
Second, there are a number of adjustable squares characterized by axial alignment structures—same as the previously described carpenter's squares and the measuring device—however not snap locking: a wing nut a thumb nut, a pull handle, etcetera needs to be manipulated before the angle can be changed. These prior art squares are characterized by the following patents:                U.S. Pat. No. 2,942,634        U.S. Pat. No. 342,849        U.S. Pat. No. 1,638,200        U.S. Pat. No. 2,775,037        U.S. Pat. No. 4,348,815        U.S. Pat. No. 4,312,133        U.S. Pat. No. 311,911        U.S. Pat. No. 306,418        U.S. Pat. No. 986,239        